The present invention relates to a method of producing an input variable for a cross-coil indicating instrument from a measurement signal, wherein the frequency of the measurement signal is converted into a frequency-proportional input current for the cross-coil indicating instrument.
The invention furthermore relates to a circuit for producing an input variable for a cross-coil indicating instrument having a measurement-signal input terminal which is connected to a period measuring device the output of which is connected to a dividing device. An output of the dividing device feeds a control variable, proportional to the frequency of the measurement signal, to an input of a processing circuit.
Such a method and circuit are known from European Patent EP 218 737 B. The period of an input signal having a frequency is determined and converted into a 10-bit digital word by formation of the reciprocal of the frequency. A processing circuit having a digital control device which operates in the known case in accordance with a PT.sub.2 -algorithm serves to compensate for variations in the frequency of the input signal. The result of the control is converted into a suitable input current for the cross-coil indicating instrument. "Input current" is a collective term for the distribution of the currents fed to the individual coils of the cross-coil indicating instrument.
Cross-coil indicating instruments are preferably used also for the indicating of non-electric variables, for instance for indicating engine rpms or speeds in a vehicle. The variables produced upon such measurements cover a relatively large frequency range so that a period measurement and an input-synchronous processing are usually required. Due to the input-synchronous processing, unsatisfactory indicating behavior results in case of low frequencies of the measurement signal. The pointer then changes its position in jerkwise fashion from time to time which is found unpleasant by an observer.